I've been going back over my "Symphony" thread and adding videos from YouTube, wherever possible. It's a pity I didn't think of this earlier!
It's nice that there are videos of live performances, or even studio performances, of many complete symphonies - such as the Beethoven cycle conducted by Karajan. On the other hand, the pickings for some symphonies are pathetically slim. I am hard put to find a video of even a complete movement from some of them. And where I can find one, it is sometimes an amateurish thing, such as a live recording where you can hear the overheads light buzzing in the background. In a few instances it looks like someone shot a video of something playing on their TV - complete with the ticking of a clock in the background! In other instances, the "video" consists of a photo or two of the conductor and/or the composer. Boring!!!
So, YouTube is not yet ready to replace your best options for reading a good symphony -- namely: hearing it performed live or listening to a complete audio recording of it.